Imaging gamma rays having energies in the range of 0.5 MeV to 2.0 MeV is useful for a variety of applications. For example, Cesium-137, which produces a 0.662 MeV gamma ray line, and Co-60, which produces 1.17 and 1.33 MeV gamma ray lines, are likely ingredients in a “dirty bomb.” The ability to monitor for the presence of these isotopes by monitoring and imaging gamma rays in their respective energy ranges would be important for anti-terrorism security purposes. Other applications for imaging gamma rays include medical imaging, non-destructive testing, mapping hot spots after nuclear accidents, and monitoring for leakage of radioactive waste.
Many conventional gamma ray imaging systems are variations of a simple pinhole camera design such as the system depicted in FIG. 1. As shown in FIG. 1, a position-sensitive gamma ray detector 1 is encased by an absorber 2 having a pinhole aperture 3. Gamma sources 4 are imaged by detecting the position at which gamma rays 5 emitted by gamma sources 4 are incident to detector 1. While a pinhole camera design offers a relatively simple solution for gamma ray imaging, the use of such a design includes significant disadvantages.
Pinhole camera designs suffer from low sensitivity since only a small portion of gamma rays 5 emitted by gamma sources 4 pass through aperture 3 to reach detector 1. Improving the sensitivity of the system by focusing gamma rays 5 is nearly impossible due to the difficulty associated with focusing gamma rays. Increasing the diameter of aperture 3 to improve sensitivity produces the undesirable consequence of also reducing imaging resolution.
In addition to sensitivity issues, gamma ray energies often create problems for imaging system designs that rely on absorbers to block a portion of the gamma rays. Specifically, gamma ray energies are usually too high to be effectively absorbed by an absorber having a reasonable thickness. Accordingly, imaging system designs that rely on absorbers are typically limited for use in imaging gamma rays having energies below a few hundred keV.
Another disadvantage associated with many conventional gamma ray imaging systems is a limited field of view. With a limited field of view, these conventional systems become directional and therefore require prior knowledge of the general position of a gamma source in order to image that gamma source. Accordingly, conventional imaging systems are not well suited for locating hidden gamma ray sources.
Finally, many conventional gamma ray imaging systems are unable to determine the direction of individual gamma rays and rely on multiple gamma rays to reconstruct a gamma source. Because of this limitation, conventional gamma ray imaging systems often require the relative positions of the imaging system and the gamma source to remain constant or in some known relation to provide useful resolution for the imaging system. This limitation also makes real-time tracking of a gamma source very difficult when either the gamma source or the imaging system is moving.
In view of the foregoing drawbacks associated with conventional gamma ray imaging systems, a need exists for a relatively small and inexpensive device for imaging gamma rays having energies in the range of 0.5 MeV to 2 MeV. Ideally, the imaging device would be capable of determining the direction of incidence of the gamma rays as well as the energy of the gamma rays.